In a local area network, a PHY device 12 in a computer 10 (FIG. 1) may be connected to another PHY device 22 in an Ethernet Switch 20 by use of a cable 32. Cable 32 typically includes four twisted-pair copper conductors 32A-32D (which may be shielded or unshielded) that carry analog signals between four transceivers 12A-12D in PHY device 12 (that in turn is coupled to MAC device 11) and four transceivers 22A-22D in PHY device 22 (that in turn is coupled to MAC device 21). Each of transceivers 12A-12D typically includes a transmit data path and a receive data path in an integrated circuit (IC) die that forms PHY device 12. The transmit data path typically includes an FEC encoder, transmit circuitry, a digital to analog converter, an analog filter, and a line driver, illustrated unlabeled in FIG. 1. Similarly, the receive data path typically includes corresponding components in a reverse order, e.g. a receive amplifier, an analog filter, an analog to digital converter, receive circuitry and an FEC decoder (also see FIG. 1).
A signal received from cable 32 by any of transceivers 22A-22D is typically weak, and any degradation affects the bit error rate (BER). Degradation of the signal during transmission across cable 32 occurs for a number of known reasons, such as echo due to reflections in cable 32, near end cross talk (NEXT) and far end cross talk (FEXT) due to the adjacency of conductors in cable 32, attenuation caused by length of cable 32, etc. Such reasons for degradation are internal to a communication channel that is formed between transceivers 12A-12D, 22A-22D by cable 32. Such internally-originated noise depends strictly on the physical characteristics of cable 32 and its connections to transceivers 22A-22D. Several prior art techniques have been developed, to suppress or cancel such internally-originated noise.
The signal transmitted through cable 32 is occasionally further degraded by noise that originates externally (“externally-generated noise”). For example, coupling of electromagnetic energy radiated by a wireless device 31, such as a walkie-talkie typically occurs in cable 32, resulting in noise therein due to electromagnetic interference (EMI). The amount of such EMI noise that gets injected into a signal in cable 32 increases as the distance reduces between wireless device 31 and cable 32. When wireless device 31 is sufficiently close, the EMI noise picked up by cable 32 can so severely corrupt a signal carried therein that a link drop occurs. The amount of EMI noise that is picked up by a signal travelling through cable 32 depends on various characteristics of cable 32, such as shielding and grounding. When cable 32 is used to transfer data at a high rate, such as 10 Gbps, the current inventor believes that prior art shielding and become insufficient e.g. because electromagnetic fields penetrate through such shielding and induce currents in cable 32. Therefore, the current inventor believes it is necessary to cancel externally-generated noise actively, as follows.